Preheat oven to 400*. In a medium bowl, beat butter and brown sugar with an electric mixer on medium speed until creamy. Beat in egg and vanilla until well blended. Stir in bananas and chocolate. Mix flour, baking powder, and salt. Add to sugar mixture and stir until blended. Drop teaspoonfuls of dough 2 inches apart onto greased cookie sheets. Bake 8-10 minutes, just until edges of cookies start to brown. Let cookies cool on sheet 2 minutes, then remove to a rack and let cool completely.